/Engine/Source/Runtime/Engine/Classes/Materials/MaterialInterface.h
Determines whether each quality level has different nodes by inspecting the material's expressions. Or is required by the material quality setting overrides.
void GetQualityLevelUsage ( [TArray](API\Runtime\Core\Containers\TArray)< bool, TInlineAllocator< [EMaterialQualityLevel::Num](API\Runtime\Engine\EMaterialQualityLevel__Type) > > & QualityLevelsUsed, EShaderPlatform ShaderPlatform, bool bCooking )